JP2567630B2 - High-fatigue strength free-cutting steel and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a free-cutting steel having excellent fatigue strength and machinability, which is suitable for mechanical structural parts such as crankshafts for automobile engines.

[Prior art]

For mechanical structural parts such as crankshafts for automobile engines, carbon steel for mechanical structures such as S50C, or steel in which free-cutting elements such as S and Pb are added is generally hot-forged, followed by quenching and tempering. Are used. Further, mechanical structural parts are required to have excellent fatigue strength because damage is caused mainly by fatigue fracture.

[Problems to be solved]

Generally, it is possible to improve the hardness to improve the fatigue strength, but on the other hand, the high hardness reduces the machinability. Also, if free-cutting elements such as S and Pb are added to improve machinability, they cause notches and reduce fatigue strength. Thus, fatigue strength and machinability are contradictory properties. is there.

In view of the above problems, the present invention aims to provide a high fatigue strength free-cutting steel having excellent performances in both the contradictory fatigue strength and machinability.

[Means for solving problems]

The first invention of the present application is a steel that is used without hot forging and without heat treatment after cooling, and has C0.30 to 0.50%.
(Weight ratio, same below), Si0.10 to 0.50%, Mn0.50 to 1.00
%, S0.04 to 0.12%, V0.05 to 0.20%, Al0.005 to 0.018%, Pb
Containing 0.05 to 0.30% and Ca 0.001 to 0.006%, the balance consisting of Fe and impurity elements, ferrite precipitating around MnS-Pb-Ca composite inclusions, and the structure consisting of ferrite-pearlite structure High-fatigue strength free-cutting steel.

The second invention is the same as the first invention, but contains a specific amount of Cr. That is, the second invention is a steel which is hot forged and is used without heat treatment after cooling, and has a C0.30 to 0.
50%, Si0.10 to 0.50%, Mn0.50 to 1.00%, S0.04 to 0.12%, V
0.05 to 0.20%, Al0.005 to 0.018%, Pb0.05 to 0.30%, Ca0.0
It should contain 01 to 0.006%, Cr 0.50% or less, the balance consisting of Fe and impurity elements, ferrite precipitating around MnS-Pb-Ca composite inclusions, and the structure consisting of ferrite-pearlite structure. It is characterized by high fatigue strength free-cutting steel.

These inventions are based on carbon steel for machine structure,
In order to secure sufficient machinability, S, Pb, and Ca were added as free-cutting elements, and the V and Cr contents were adjusted.

That is, S has MnS and Pb has the effect of improving the chip disposability during turning and drilling, and Ca has the effect of improving the tool life during turning.
However, if these are simply added, these become "notches", which causes fatigue fracture to start and reduce fatigue strength. Therefore, in order to prevent the notch effect due to these elements, various researches have been carried out.
The inventors have invented that the notch effect can be prevented by covering the periphery of the MnS-Pb-Ca composite inclusion with ferrite having high ductility. Further, as a notch preventing means, the heat treatment of this free-cutting steel is 800-600 after hot forging of the steel having the composition of the present invention, not by conventional quenching and tempering.
It was decided to cool at a cooling rate between 1 ° C and 100 ° C / min. By this cooling, in the process of transformation from austenite structure to ferrite-pearlite structure,
Fine ferrite is precipitated around the above MnS-Pb-Co composite inclusions.

Generally, in carbon steel for machine structure such as S50C, the microstructure as hot forged is coarse ferrite.
It has a pearlite structure and has lower strength and lower fatigue strength than the quenched and tempered materials. However, by adding V to this, the strength and fatigue strength are increased, and by precipitating ferrite around the above-mentioned complex inclusions, a fine ferrite-pearlite structure can be obtained even with hot forging. . Therefore, it is possible to secure the fatigue strength equal to or higher than that of the quenched and tempered materials. Furthermore, steel with a ferrite-pearlite structure as hot forged has a much better machinability than hardened and tempered materials.

 Next, the reasons for limiting each element will be described.

First, C is required to be 0.30% or more in order to secure the strength required for structural steel. However, when a large amount of C is contained, the amount of ferrite decreases, making it difficult to precipitate ferrite around the complex inclusions, and the toughness also decreases. Therefore, the upper limit was set to 0.50%.

Si needs to be at least 0.10% as a deoxidizing auxiliary agent, but if a large amount of Si is contained, the toughness decreases, so the upper limit was made 0.50%.

Mn needs to be at least 0.50% to secure the formation of MnS and the ferrite-pearlite structure, but the upper limit was set to 1.00% because a large amount of Mn reduces the machinability.

As described above, S is required as MnS for improving the machinability and at least 0.04% as ferrite precipitation nuclei. However, if a large amount is contained, hot workability deteriorates, so the upper limit was made 0.12%.

V has the effect of precipitating as carbides in the ferrite structure during cooling after forging, improving the strength, and at least
0.05% is necessary. However, even if a large amount is added, the corresponding effect is small and the cost is high, so the upper limit was made 0.20%.

Al needs to be at least 0.005% as a deoxidizing agent, but the upper limit was set to 0.018% because the formation of Al ₂ O ₃ deteriorates the machinability, especially the tool life.

Pb must be at least 0.05% to improve machinability. However, if it exceeds 0.30%, the improvement in machinability saturates and the cost increases, so the upper limit was made 0.30%.

Ca has the effect of coating the surface of the cutting tool and improving the tool life, and also rounds the shape of MnS to prevent notches. Therefore, at least 0.001% is necessary. However, if it exceeds 0.006%, the effect is saturated, so the upper limit was made 0.006%.

Further, Cr relating to the second invention is for further improving the strength. However, if contained in a large amount, the machinability deteriorates, so the upper limit was made 0.50%.

In addition, in the above, the amount of C and Mn is 0.5 or more, the hardenability improves, and it becomes difficult for ferrite to precipitate around the inclusions. The above is preferable.

In addition, as described above, the first and second invention steels are
By cooling between 00 ℃ and 600 ℃ at a temperature of 1-100 ℃ per minute, ferrite was precipitated around the MnS-Pb-Ca inclusions. A fine ferrite-pearlite structure is obtained, and it exhibits excellent fatigue strength and machinability. The cooling conditions of 800 ° C. to 600 ° C. after the above hot forging are extremely important for obtaining the excellent properties of the present invention. If the cooling rate is less than 1 ° C / min, the hardness of the free-cutting steel obtained will be low and excellent fatigue strength will not be obtained, while 100
If the temperature exceeds ℃ / min, the hardness becomes high, and ferrite does not precipitate around the above MnS-Pb-Ca composite inclusions, resulting in a coarse ferrite-pearlite structure, resulting in poor machinability and durability (see Example 2). ).

In addition, as a method for producing the first invention steel, C0.30 to 0.50%, Si0.10 to 0.50%, Mn0.50 to 1.00%, S0.04
~ 0.12%, V0.05 ~ 0.20%, Al0.005 ~ 0.018%, Pb0.05 ~ 0.
After hot forging a steel containing 30% of Ca and 0.001 to 0.006% of Ca, with the balance being Fe and impurity elements, cooling at a rate of 1 to 100 ° C per minute between 800 ° C and 600 ° C Thus, there is a method for producing high-fatigue strength free-cutting steel characterized in that the steel structure is a ferrite-pearlite structure in which ferrite is precipitated around MnS-Pb-Ca composite inclusions.

As a result, the first invention steel having the above-mentioned excellent effects can be obtained.

Further, as a method for producing the second invention steel, C0.30 to 0.50%, Si0.10 to 0.50%, Mn0.50 to 1.00%, S0.04
~ 0.12%, V0.05 ~ 0.20%, Al0.005 ~ 0.018%, Pb0.05 ~ 0.
Contains less than 30%, Ca 0.001 to 0.006%, Cr 0.50%, balance
After hot forging steel consisting of Fe and impurity elements, cooling the steel structure between 800 ℃ and 600 ℃ at a rate of 1 to 100 ℃ per minute, the structure of the steel is changed to MnS-Pb-Ca composite inclusions. There is a method for producing a high fatigue strength free-cutting steel, which is characterized by forming a ferrite-pearlite structure in which ferrite is precipitated around.

As a result, the second invention steel having the excellent effects as described above can be obtained.

[Action and effect]

In the first invention, S, Pb, and Ca are added at the same time to obtain MnS-Pb-
By forming Ca composite inclusions and improving the machinability extremely, and by covering the surroundings with highly ductile ferrite, the notch effect that reduces the fatigue strength is prevented. Further, in the present invention, the ratio of the components of the present invention together with these three components is adjusted as described above, so that it is possible to provide the steel property which is excellent in strength and fatigue strength and is also excellent in free-cutting property. Therefore, the first invention steel has high practicability as a hot forged part such as a crankshaft.

Further, the second invention is one in which Cr is positively added to the first invention, and in addition to obtaining the same effects as the first invention, it is possible to exhibit more excellent strength.

Further, according to the above manufacturing methods, it is possible to obtain the high fatigue strength free-cutting steel having the excellent effects as described above.

〔Example〕

First Example Next, the features of the invention steel according to the present application will be described by way of examples in comparison with a comparative steel and a conventional steel.

Table 1 shows the chemical composition of these test steels. In Table 1, A to F steels are the present invention steels, among which A to D steels are the first invention steels, E and F steels are the second invention steels, GM steels are comparative steels, N and O steels. Is conventional steel. In the table, the part without indication of Cr content indicates that it contains less than 0.2% of Cr impurity.

Further, in the above, the A to M steels are forged as they are, and the N and O steels are forged and then quenched and tempered.

The above-mentioned as-forged test steel was manufactured by casting a 300 kg ingot in a high-frequency melting furnace and then forging it to a diameter of 100 mm.
After heating to 50 ° C, it was forged to a diameter of 65 mm at 1200 to 1100 ° C, and then air-cooled at a cooling rate of 25 ° C / min.
Further, the test steel that was quenched and tempered after the above forging was prepared by oil quenching the above 65 mm diameter material at 880 ° C and then at 530 ° C.

An evaluation test was conducted on each of the above-mentioned test steels, and the results are shown in Table 2.

In the evaluation test, the tensile test piece was based on JIS No. 4, and the hardness was measured at those chucks. Also, in the table, the fatigue limit is 10
^This is the value after ⁷ cycles, and the durability ratio is the durability indicated by the ratio of "fatigue limit / tensile strength". Also, turning property is TiN
Using coated carbide tip tool, feed rate 0.20mm /
Rotation, depth of cut 2.0 mm, no lubrication, cutting speed of 200 m / min, and the evaluation value was the time (min) until flapping wear 0.2 mm. For drill workability, a SKH906 mmφ straight drill was used, feed rate 0.11 mm / revolution, rotation speed 800
The evaluation was carried out under the conditions of rpm and no lubrication, and the drilling distance (m) until the drill was completely dull. In addition, in the column of microstructure in the table, "fine F / P", "coarse F / P"
“P” refers to a fine or coarse ferrite / pearlite structure. "Incomplete" refers to an incompletely quenched and tempered structure.

As is known from the above both tables, the steel of the present invention has hardness Hv21.
0 or more, 10 ⁷ times Fatigue limit 33kg f / mm ² , tensile strength 70kg f / mm ² or more, durability ratio 0.47 or more are all satisfied, and machinability is both turning 40 minutes and drilling machinability 5m or more. However, it can be seen that it has excellent fatigue strength and machinability. Further, it can be seen that the microstructures of the steel of the present invention all have fine ferrite and pearlite.

Also, in the comparative steel, the chemical component S is about the degree of impurities,
No. G, H and I steels containing no Pb or Ca have low machinability, and J or M steels with high C content or high Mn content have high hardness but durability ratio and machinability. Is low. In addition, K steel, which does not contain V as a chemical component, has excellent turning properties, but fatigue strength,
The durability ratio is low. In addition, L steel with high Al has extremely low turning property.

On the other hand, conventional N steel, which has been quenched and tempered after forging, has high fatigue strength and durability ratio, but has low machinability because it does not contain free-cutting elements. O steel does not contain V and has been subjected to quenching and tempering treatments, but its notch effect has a low durability ratio, and simply adding free-cutting elements cannot achieve the target characteristics.

Second Example For the steel having the same composition as the A steel shown in the first example,
The cooling conditions after forging were changed, and the obtained steel was evaluated in the same manner as in the first example. The cooling conditions are shown in Table 3 and the evaluation results are shown in Table 4.

As is known from both tables, the steel of the present invention is 5 or 80 ° C /
It can be seen that even at a cooling rate of minutes (No. A2, A3), high fatigue strength and excellent machinability are exhibited, and a wide range of cooling rates can be selected. On the other hand, the cooling rate is 13
As fast as 0 ° C / min, A1 has a high hardness, and ferrite does not precipitate around inclusions, resulting in a coarse ferrite-pearlite structure and poor machinability and durability. Furthermore, the cooling rate is 0.8
C4 / min, which is slow, has low hardness and poor fatigue strength.

Therefore, the cooling rate must be 1 to 100 ° C / min.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keiyo Wakimon 1 Wanowari, Arao-cho, Tokai-shi, Aichi Aichi Steel Works (56) References JP 61-291955 (JP, A) JP 59 -219454 (JP, A)

Claims (4)

(57) [Claims] 1. A steel which is hot forged and is used without heat treatment after cooling, wherein C0.30 to 0.50% (weight ratio, the same hereinafter), Si0.10 to 0.50%, Mn0.50 to 1.00 %, S 0.04 to 0.12%, V
0.05 to 0.20%, Al0.005 to 0.018%, Pb0.05 to 0.30%, Ca0.0
01-0.006%, the balance consists of Fe and impurity elements, ferrite precipitates around MnS-Pb-Ca composite inclusions,
High-fatigue strength free-cutting steel characterized by having a structure of ferrite / pearlite structure. 2. Steel for hot forging, which is used without heat treatment after cooling, having C0.30 to 0.50%, Si0.10 to 0.50%,
Mn0.50 ~ 1.00%, S0.04 ~ 0.12%, V0.05 ~ 0.20%, Al0.005
~ 0.018%, Pb0.05 ~ 0.30%, Ca0.001 ~ 0.006%, Cr0.50%
Contains the following, the balance consisting of Fe and impurity elements, ferrite precipitates around MnS-Pb-Ca composite inclusions,
High-fatigue strength free-cutting steel characterized by having a structure of ferrite / pearlite structure. 3. C0.30 to 0.50%, Si0.10 to 0.50%, Mn0.50 to
1.00%, S0.04 to 0.12%, V0.05 to 0.20%, Al0.005 to 0.018
%, Pb0.05-0.30%, Ca0.001-0.006%, the balance
After hot forging steel consisting of Fe and impurity elements, cooling the steel structure between 800 ℃ and 600 ℃ at a rate of 1 to 100 ℃ per minute, the structure of the steel is changed to MnS-Pb-Ca composite inclusions. A method for producing a high-fatigue strength free-cutting steel, characterized by having a ferrite-pearlite structure in which ferrite is precipitated around. 4. C0.30 to 0.50%, Si0.10 to 0.50%, Mn0.50 to
1.00%, S0.04 to 0.12%, V0.05 to 0.20%, Al0.005 to 0.018
%, Pb0.05 to 0.30%, Ca0.001 to 0.006%, Cr0.50% or less, the balance of which is Fe and impurity elements. High fatigue strength free-cutting steel characterized by forming a ferrite-pearlite structure in which ferrite is precipitated around MnS-Pb-Ca composite inclusions by cooling at a rate of 1 to 100 ° C per minute Manufacturing method.